.div1,
.div2,
.div3 {
    width: 100px;
    height: 100px;
    border: 1px solid red;
    margin-left: 10px;
    text-align: center;
    line-height: 100px;
    float: left;
}
.div1::after {
    content: '666';
}
.div1::before {
    content: '777';
    color: red;
}
/* .big {
    height: 104px;
} */
/* .div4 {
    clear: both;
} */
.big::after {
    display: block;
    content: '';
    clear: both;
}